HUMAN PHYSIOLOGY: INTRODUCTION 299 



substances, if they are capable of mixing. That is, the 

 molecules of one liquid become thoroughly intermingled 

 with the molecules of the other, as when syrup is poured into 

 water. This is diffusion. 



If two such liquids are separated by an animal membrane, 

 the molecules of each will pass through the membranes 

 so that in time the solutions on each side will be equally 

 diffused. This passage of the molecules of liquid through 

 an animal membrane is known as osmosis. . The " attrac- 

 tion" exerted mutually by the liquids is called osmotic pres- 

 sure. Osmotic pressure varies with the density and tem- 

 perature of the solutions. Osmosis and osmotic pressure 

 are facts of great importance in animal physiology as we 

 shall see when we study the nutrition of the cells. 



Procure from the butcher a small bladder; moisten it and fill it with 

 sugar solution. Insert a tube into the opening and tie the neck of the 

 bladder tightly around the tube. Immerse this in a dish of water and 

 note the result. The energy with which the water enters the solution of 

 greater density is due to osmotic pressure. 



Relative value of common foods. — Our meals consist 

 usually of a "mixed diet," of fruit, nuts, cereal and eggs, 

 or perhaps bread, meat, and vegetables. Experience has 

 shown that a mixed diet is better suited to the appetite 

 and to the needs of the body than a single food material. 

 Eggs, milk and bread are nutritious because they contain 

 almost no waste. Meat is valuable for its high percentage 

 of proteid. Potatoes and other vegetables are valuable for 

 their high percentage of carbohydrates. Some foods are 

 valuable for their large percentage of water. 



Daily diet. — Certain experiments have been made both in 

 Europe and America to determine a "standard diet" — the 

 amount of food that should be consumed in order to 

 preserve the health. This amount varies of course with 

 the occupation,- the sex and the age of the individual 

 and with the climate. 



